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United States Patent |
5,610,633
|
Eltgen
|
March 11, 1997
|
Agent for magnetographic printers and use of such an agent
Abstract
The present invention relates to an agent for magnetographic printers that
includes a plurality (k) of elementary magnetic layers, having relatively
hard hysteresis cycles with a threshold effect and marked saturation, of
which the coercivity and/or thickness of the layers varies as a function
of the position of the layer on a soft magnetic substrate.
Inventors:
|
Eltgen; Jean-Jacques (Danjoutin, FR)
|
Assignee:
|
Nipson (Belfort, FR)
|
Appl. No.:
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277743 |
Filed:
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July 20, 1994 |
Foreign Application Priority Data
Current U.S. Class: |
346/74.2; 347/131 |
Intern'l Class: |
B41J 002/415; B41J 002/385; G11B 009/00 |
Field of Search: |
346/74.2
430/31
347/111,112,131
428/900,928
360/115,113
324/252
|
References Cited
U.S. Patent Documents
4798622 | Jan., 1989 | Pingaud | 252/62.
|
5147732 | Sep., 1992 | Shiroishi et al. | 428/928.
|
5153796 | Oct., 1992 | Gooch | 360/115.
|
5304975 | Apr., 1994 | Saito et al. | 360/113.
|
5386332 | Jan., 1995 | Jagielinski et al. | 428/900.
|
Foreign Patent Documents |
2600178 | Dec., 1987 | FR.
| |
8204344 | Dec., 1982 | WO.
| |
Primary Examiner: Fuller; Benjamin R.
Assistant Examiner: Gordon; Raquel Yvette
Attorney, Agent or Firm: Kerkam, Stowell, Kondracki & Clarke, P.C., Kondracki; Edward J.
Claims
I claim:
1. An agent for magnetographic printers, comprising:
a plurality (k) of elementary magnetic layers disposed on a soft magnetic
substrate, said magnetic layers having relatively hard hysteresis cycles
with a defined writing threshold and saturation magnetization, wherein at
least one of a coercivity and a thickness of said layers varies as a
function of a depth of the layer on said soft magnetic substrate.
2. The agent of claim 1, wherein said layers have a same thickness, each of
said layers having a different coercivity, said coercivity increasing in a
direction in which the layers are deposited in the soft magnetic
substrate.
3. The agent of claim 2, wherein said agent is used with demagnetization
lines that have identical slopes for each said layer.
4. The agent of claim 1, wherein said layers have a same coercivity and
decreasing thickness, said thickness decreasing in a direction in which
the layers are deposited in the soft magnetic substrate.
5. The agent of claim 4, wherein said agent is used with demagnetization
lines that have decreasing slopes in the direction in which the layers are
deposited in the soft magnetic substrate.
6. The agent of claim 2, wherein variation of coercivities is obtained by
doping with at least one of hypophosphites and adjustment of current
densities of the deposit.
7. The agent of claim 3, wherein variation of coercivities is obtained by
doping with at least one of hypophosphites and adjustment of current
densities of the deposit.
8. The agent of claim 1, wherein the layers are deposited by vacuum
sputtering.
9. The agent of claim 2, wherein the layers are deposited by vacuum
sputtering.
10. The agent of claim 3, wherein the layers are deposited by vacuum
sputtering.
11. The agent of claim 4, wherein the layers are deposited by vacuum
sputtering.
12. The agent of claim 5, wherein the layers are deposited by vacuum
sputtering.
13. The agent of claim 6, wherein the layers are deposited by vacuum
sputtering.
14. The agent of claim 7, wherein the layers are deposited by vacuum
sputtering.
15. The agent of claim 1, wherein the layers are separated by fine
nonmagnetic layers.
16. The agent of claim 2, wherein the layers are separated by fine
nonmagnetic layers.
17. The agent of claim 3, wherein the layers are separated by fine
nonmagnetic layers.
18. The agent of claim 4, wherein the layers are separated by fine
nonmagnetic layers.
19. The agent of claim 5, wherein the layers are separated by fine
nonmagnetic layers.
20. The agent of claim 6, wherein the layers are separated by fine
nonmagnetic layers.
21. The agent of claim 7, wherein the layers are separated by fine
nonmagnetic layers.
22. The agent of claim 8, wherein the layers are separated by fine
nonmagnetic layers.
23. A method of printing using an agent, said agent including a plurality
(k) of elementary magnetic layers disposed on a soft magnetic substrate
and having relatively hard hysteresis cycles with a defined writing
threshold and saturation magnetization, wherein at least one of a
coercivity and a thickness of said layer varies as a function of a depth
of the layer on said soft magnetic substrate, the method comprising the
steps of:
providing a plurality of writing heads;
printing n.sup.2. k grey shades on a sheet of paper, wherein n is a number
of dots used to define a pixel size and wherein said soft magnetic
substrate is coated with k layers, enabling grey levels to be defined as a
function of different thicknesses of the layers or of different coercivity
of the layers;
applying a different writing threshold to each writing head of said
plurality of writing heads, thereby obtaining k grey levels per elementary
dot defined by different masses of toner retained per unit of surface
area; and
printing, with the plurality of heads having n.times.n points.
24. The method of claim 23, wherein the number n of dots equals 2, and the
number k of layers equals 3.
25. The method of claim 23, wherein the number n of dots equals 2, and the
number k of layers equals 2.
Description
FIELD OF THE INVENTION
The present invention relates to an agent for magnetographic printers and
its use for printing different shades of grey on a print medium.
BACKGROUND OF THE INVENTION
In the current state of known embodiments, magnetography is essentially a
digital technique, which makes it possible to have grey scales but to the
detriment of the image resolution. Hence for a given addressability, the
dots making up the image are either magnetically unsaturated or as close
as possible to saturation. As a result, the reflectance of the positions
in the mapping grid assume only two values, white and black. Under these
conditions, a scale of grey shades can be made up only by using a picture
element or "pixel" composed of several dots, by a method sometimes called
"dithering". In general, the pixel is made up of a square n.times.n in
size, thus offering n.sup.2 different levels of grey, as shown in FIG. 5.
Digital resolution of the greys by using a pixel with n.times.n dots has
the grave disadvantage of reducing the image resolution R in the ratio n.
For example, for an addressability that represents the spatial density of
the theoretical grid of the addressable dots, that is, 240 dots per inch
(dpi) that can be written upon and a 4.times.4 pixel allowing 16 grey
levels, The image resolution drops to 60 pixels per inch (ppi), which is
manifestly too low for the eye to correctly integrate the reflectances of
the various dots of the pixel so as to obtain the mean value thereof. If
one wishes to preserve an image resolution of 240 ppi, then the
alternative consists of using addressability that is four times greater,
that is, 960 dpi. Obtaining such densities cannot be done without
encountering serious problems in making the requisite writing head bars in
high-speed parallel printers.
SUMMARY OF THE INVENTION
Accordingly, the object of the present invention is to propose an agent
that makes it possible to generate images having a sufficient number of
shades of grey without degrading the addressability or resolution of the
image.
This object is attained by an agent for magnetographic printers that
includes a plurality (k) of elementary magnetic layers, having relatively
hard hysteresis cycles with a threshold effect and marked saturation,
wherein the coercivity and/or thickness of the layers varies as a function
of the position of the layer on a soft magnetic substrate.
In another embodiment, the agent for magnetographic printers includes a
plurality (k) of elementary magnetic layers, having relatively hard
hysteresis cycles with a threshold effect and marked saturation, each
having the same thickness and different coercivity, the coercivity
increasing in the direction of the layers deposited in depth in the soft
magnetic substrate.
In another particular feature, the agent is used with demagnetization lines
having identical slopes for each layer.
In another embodiment, the layers are of the same coercivity and have
decreasing thickness in the direction of the layers deposited in depth in
the soft magnetic substrate.
In another feature, the agent is used with demagnetization lines having a
decreasing slope in the direction of the layers deposited in depth in the
soft magnetic substrate.
In another feature, the variation of the coercivities is obtained by doping
with hypophosphites and/or adjustment of the current densities of the
deposit in the case of layers obtained by electrolytic deposit of
cobalt-nickel alloy.
In another feature, the layers are deposited by vacuum sputtering.
In another feature, the different layers are separated by fine nonmagnetic
layers, to facilitate the deposit of the next layer and to reduce the
magnetic coupling among the various layers.
Another object of the invention is to propose the use of the agent of the
invention to make printing with multiple grey shades without increasing
the number of writing heads of the writing head bar.
This object is attained by using a soft magnetic substrate coated with an
agent with k layers, making it possible, as a function of the different
thickness of the layers or the different coercivity of these layers, to
obtain k grey levels defined by different masses of toner retainer per
unit of surface area, and a head for writing pixels with n.times.n points.
Thus providing n.sup.2 .multidot.(k) grey shades on a sheet of paper.
As an example of use, the number n of dots may equal 2, and the number k of
layers may equal 3, making 12 final grey levels.
In other example, the number of dots may equal 2, and the number k of
layers may equal 2, making 8 final grey levels.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features and advantages of the present invention will become more
apparent from the ensuing description of various embodiments of the
invention, given by way of non-limiting example, with the aid of the
accompanying drawings, in which:
FIG. 1 shows an embodiment of an agent with superimposed layers and writing
poles;
FIG. 2 shows hysteresis cycles of the various layers of the agent;
FIG. 3 shows use of a substrate with three layers which is associated with
a pixel with four dots (2.times.2), making twelve grey levels possible;
FIG. 4 shows a non-exhaustive table of possible configurations.
FIG. 5 shows examples of images with 16 grey shades obtained by the method
of the prior art;
FIG. 6 shows curves of residual magnetization of elementary dots of the
various layers of the agent;
FIG. 7 shows total magnetostatic energy of a dot on a multilayer agent as a
function of the writing field of the head;
FIG. 8 shows a curve determining the residual magnetization of a layer
based on the geometry of the dot defining the demagnetizing factor and the
slope of the demagnetization curve;
FIG. 9 shows curves that make it possible to determine the threshold fields
for an agent with layers of the same thickness but with magnetization at
increasing saturation in the direction of the depth of the substrate.
FIG. 10 shows hysteresis curves for an agent with layers of decreasing
thickness from the surface toward the substrate and with magnetization at
constant saturation for each layer;
FIG. 11 shows hysteresis curves enabling the determination of threshold
fields for an agent made up of layers with the same saturation
magnetization and the same thickness.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 shows a soft magnetic substrate (S), which for example can make up
the drum of a magnetographic machine, of the kind known from European
Patent Application EP 082 742, or may be continuous tape, or any other
agent for a print medium or the like.
Disposed on the substrate (S) are several exemplary magnetic layers C1, C2,
C3. Three is the number used in the illustrative example; however, it is
understood that this is merely for illustrative purposes and in no way
limits to the scope of the invention. These layers are optimally separated
by fine nonmagnetic layers to facilitate their deposition. Each layer, as
shown in FIG. 2, has a different saturation magnetization (Msi) and a
different coercive field (Hci), with the coercive fields and
magnetizations having increasing values from one layer to another in the
direction of displacement of the writing surface toward the core of the
substrate.
In addition, as shown in FIG. 2, each individual layer (Ci) has
conventional magnetic properties. Particularly a relatively "hard"
hysteresis cycle having a threshold effect and marked saturation. Thus the
hysteresis cycle of the layer C3 has a coercive field (Hc3) greater than
the coercive field (Hc2) of the cycle of the layer C2. Similarly, the
saturation magnetization of the layer C3, Ms3, is greater than the
saturation magnetization Ms2 of the layer C2 shown in FIG. 2. The coercive
fields (Hci) of the different layers (Ci) are clearly distinct, thereby
defining a succession of intervals of disconnected fields. In each of
these intervals taken separately, each of the layers is either not
significantly written, or practically saturated.
The coercive field (Hci) is defined by the value for which the curve
representing the hysteresis cycle intersects the axis of the field (H),
and the saturation field (Hsi) is defined by the value for which the two
curves forming the hysteresis cycle come together again as represented by
Hs in FIG. 8.
This value (Hsi) can be expressed as a function of the coercive field (Hci)
by the equation Hsi=.lambda.i.multidot.Hci.
Associated with each saturation field (Hsi), as a function of the
demagnetizing field (Hdi), is a writing threshold field (Hwi) that
indicates the values at which writing takes place or below which there is
no magnetization, that is determined by the equation Hwi=Hsi+Hdi.
The demagnetizing field is linked with the geometry of the elementary dot
(thickness in proportion to the magnetized surface area) and with the
magnetization taken up by the layer, in accordance with the equation
Hdi=Di Msi, where Di is the demagnetization factor. This equation defines
a slope direction -1/Di, called the demagnetization direction, in the
plane (H, M).
When the demagnetizing field reaches the value of 0 (zero), as a function
of its demagnetizing factor (Di) which is represented by the
demagnetization curve, which in fact is a straight-line slope -1/Di, the
magnetic layer (Ci) preserves a residual magnetization (Mdi), which is
represented by the projection on the axis of the magnetizations of the
point of intersection of the hysteresis cycle with the demagnetization
line of slope -1/Di, passing through the center (00) of the coordinate
system. Hence for a given coercive field (Hci) and for given magnetic
properties of the layer (Ci), it would have to be exposed to a
magnetization field greater than the writing threshold field (Hwi) in
order then, following a demagnetization, to produce a significant residual
magnetization (Mdi). The residual magnetization (Mdi) can be expressed by
the following equation:
M.sub.di =H.sub.ci (dM/dH).sub.0i /[D.sub.i +(dM/dH).sub.0i ]
For cycles assumed to be perfectly square and hence which approach the
cycles shown in FIG. 2; because .lambda..sub.i is practically equal to 1
(in fact .lambda.i depends only on the squareness of the intrinsic cycle,
and not on the geometry of the dots), (dM/dH).sub.0i is much higher than
1, which expresses the fact that the slope in the vicinity of the dot H=Hc
is pronounced. Finally, for very flat dots, that is, of slight thickness
relative to their surface area, Di is practically equal to 1, hence the
simpler expression:
H.sub.wi .apprxeq.H.sub.ci +M.sub.si
M.sub.di .apprxeq.Hc.sub.i
Accordingly, by exposing a layer (Ci) to a field greater than its writing
threshold field (H.sub.wi), the pixel will, depending on the layer used,
have a residual magnetization (M.sub.di) which corresponds to the residual
magnetizations shown in FIG. 6. Since the substrate is made up of a
succession of different layers with a different coercive field, the
residual magnetization of the dot on a multilayer substrate will
correspond in actuality to the curve of FIG. 7 as a function of the
writing field levels, thus making it possible for each dot to create four
possibilities of grey level representation. These possibilities of gray
level representation correspond to the residual magnetization values
represented by all the layers, when they are exposed to different
threshold fields, as shown in FIG. 7.
Such an approach allows the use of writing heads with pronounced writing
field dispersion for a given excitation current, since this in turn means
creating an "energy of the dot/writing field" characteristic that is
always strongly nonlinear and has a plurality of intermediate saturation
thresholds. The version presented in FIGS. 1 and 2 is summarized in FIG.
9, where one can confirm that the three layers of the same thickness used
with identical demagnetization line slopes and hence identical
demagnetization coefficients must have quite different saturation
magnetizations Msi, to enable major differentiation of the threshold
fields Hwi. These different writing threshold fields will thus make it
possible to achieve three grey levels at the level of the elementary dot.
In addition, one can confirm that in this embodiment, the residual
magnetizations Mdi increase from the surface layers toward the deeper
layers. The variation in the coercivities and consequently the saturation
magnetizations is obtained by doping with hypophosphites and/or adjusting
the current densities of deposition in the case of electrodeposited
magnetic layers made up of cobalt-nickel-phosphorous.
The relationships that have been seen above can be optimized by making
measurements with real cycles. In that case, the squarenesses of the
hysteresis cycles determine the coefficient .lambda.i, and generally this
coefficient is on the order of two or three as well as the slopes of the
dM/dH lines in the vicinity of the coercive field (Hc). The thickness of
the various layers compared with the surface area S of the written dots,
which in turn depends on the cross section of the writing pole, makes it
possible to estimate the demagnetization coefficients Di of these layers
and from that to draw the slope -1/Di of the demagnetization function
lines. The tracing of these lines then shows the residual magnetizations
of the various layers and the writing threshold fields Hwi corresponding
to the various levels. The objective is to obtain sufficiently wide zones
between the various thresholds Hwi, as in the first embodiment described
above.
The embodiment of FIG. 10 shows another embodiment of the invention, in
which the magnetic layers C1, C2, C3 decrease from the surface toward the
substrate S, while the demagnetization factors increase in the same
direction. Consequently the slopes (-1/Di) of the function lines decrease
from the surface toward the substrate S. The divergence among the function
lines accentuates the intervals between the writing threshold fields
(Hwi), even in the case of layers with the same saturation magnetization.
If, in addition, the saturation magnetizations are different for each
layer, then the differentiation among threshold fields is improved.
Furthermore, this variant also has the effect of making the residual
magnetizations (Mdi) closer together. This embodiment is relatively easily
achieved, because a succession of coercive fields (Hci) is easily obtained
by doping with hypophosphites and/or with adjustments of the current
densities of deposit in the case of electrodeposited layers of
cobalt-nickel-phosphorous (Co-Ni-P). Conversely, obtaining a succession of
saturation magnetizations (Msi) covering a wide zone, as in the case of
FIG. 9, proves to be more difficult.
The use of layers with the same saturation magnetization (Msi) and the same
thickness, shown in FIG. 11, which has the consequence of producing
identical function line slopes, does not enable obtaining sufficient
distances between threshold fields Hwi. In fact, if the distances between
threshold fields (Hwi) are too small, then despite the major dispersion of
the fields produced by the writing heads, the thresholds will not be
sufficient to achieve grey levels that one can easily distinguish.
While this invention has been described in conjunction with specific
embodiments thereof, it is evident that many alternatives, modifications
and variations will be apparent to those skilled in the art. Accordingly,
the preferred embodiments of the invention, as set forth herein, are
intended to be illustrative, not limiting. Various changes may be made
without departing from the spirit and scope of the invention as described
herein and defined in the appended claims.
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